Structure Of The Cantal Strato-Volcano, French Central Massif, From Electric, Magnetotelluric Soundings And Aeromagnetic Data

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Journal Article: Structure Of The Cantal Strato-Volcano, French Central Massif, From Electric, Magnetotelluric Soundings And Aeromagnetic Data

Abstract
The Neogene composite volcano of Cantal is the largest volcano in Europe. Its area (2500 km2) is larger than that of Etna (1200 km2). It was formed between 23 and 3 Ma ago. An aeromagnetic survey, in addition to electrical (ES) and magnetotelluric soundings (MTS) carried out in recent years have made it possible to describe the structure of this volcano. The terrain effects of the various pyroclastics and lavas that make up the volcanic edifice have been computed and removed from the aeromagnetic data; the analysis of the resulting terrain-corrected residual anomalies gives clear evidence for the presence of three internal or deep-seated magnetized bodies which are also revealed by the ES and MTS investigations: 1. (1) The main hypostructure corresponds to a large volcano-tectonic trough filled with low-resistivity (10-5 ωm) and moderately magnetized (1 A/m) volcanic material. However, a part of the trough is filled with non-magnetic formations and it may be an Oligocene sedimentary graben cut across by the present volcanic trough. 2. (2) The hypovolcanic intrusion (monzonites and gabbros) of the Jordanne upper valley was previously known from an outcrop and a borehole (260 m). Its large extent (15 km2) is shown by magnetic study as well as by ES and MTS. According to the magnetic interpretation, its thickness would be 300 m. 3. (3) The nature of the structure inside the Plomb du Cantal is still uncertain. In view of its high resistivity and high magnetization, it may be an unexposed pile of lavas inside the cone or a large intrusion. In the area where a caldera affects the volcanic cone, ES and MTS underline the importance of a hydrothermal process (propylitization) that lowers and homogenizes the resistivity (p = 30 ω m) of all volcanic rocks and probably that of the uppermost part of the crystallophyllian basement.

Authors 
M. Aubert, A. Dupis, J. F. Lenat, J. Roux and G. Senaud








Published Journal 
Journal of Volcanology and Geothermal Research, 1982





DOI 
Not Provided
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Citation

M. Aubert,A. Dupis,J. F. Lenat,J. Roux,G. Senaud. 1982. Structure Of The Cantal Strato-Volcano, French Central Massif, From Electric, Magnetotelluric Soundings And Aeromagnetic Data. Journal of Volcanology and Geothermal Research. (!) .